The invention relates to a process for treating diesel engine exhaust gases comprising nitrogen oxides (NOx) and hydrocarbons (HC) by selective catalytic reduction of the nitrogen oxides with ammonia or a compound decomposable to ammonia as a reducing agent over an SCR catalyst based on a molecular sieve. The properties of the catalyst used are such that the hydrocarbons present in the exhaust gas are kept away from the catalytically active sites in the catalyst over which the reactions take place by the molecular sieve-like action of the zeolite present in the catalyst. This prevents HC-related degradation and aging effects of the SCR catalyst and achieves a considerable improvement in nitrogen oxide conversions in HC-containing exhaust gas.
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